I had similar symptoms to you 2 weeks ago, but might not be the same problem so don't take this as gospel.

Car was parked up all week and I was faffing around with USB sticks and attempting to get the car to read them. Finally succeeded, tried to start the car to go to a meet, engine turned over very slowly before finally dying. Tried charging the battery (cheap charger) and it turned over healthily but refused to catch. Bought new battery, fitted it (no issues about registering - I just plug n played) and still the same: would turn over healthily but not start.

I got the car towed to a mate's garage where the fault reader diagnosed it, although it needed BMW themselves to recode it correctly.

Problem was caused by the battery getting so low on charge (as I repeatedly cranked it over attempting to start it with the old battery) that it caused problems with the "EWS manipulation protection" system. In layman's terms, the car lost the ability to recognise the coding from the key hence the immobiliser system was kicking in and stopping it from starting. It would turn over but not fire.